The present invention relates to a power transmission system for a motor vehicle, and more particularly to a differential operation restricting (a limited slip) system with a hydraulically-operated multiple-disk friction clutch in which hydraulic pressure applied to the clutch is controlled for variably changing transmission torque to right and a left driving wheels in accordance with driving conditions of the motor vehicle and the surface conditions of the road.
Japanese Patent Application Laid-Open No. 62-143719 discloses a system in which a hydraulically operated multiple-disk friction clutch is provided between a differential case and a side gear of a differential for producing torque to restrict differential operations. Hydraulic pressure for the clutch is controlled for changing the bias ratio of a limited slip differential so as to improve the steerability of the motor vehicle.
The same lubricant for the multiple-disk friction clutch as that of an automatic transmission is used for actuating and lubricating the clutch, since hydraulic fluid has a good friction characteristic and a low oil viscosity. On the other hand, a final reduction gear in a differential housing provided on the outside of the differential case is composed of a hypoid gear. In such a differential, an extreme pressure agent is provided which contains additives to improve the ability to adhere to the surfaces of metals under high bearing pressures. A lubricant is subject to cause the multiple disks of the clutch to stick and to slip because of unstable friction characteristics, which causes unpleasant vibration and noise in the vehicle body.